Inverter

ABSTRACT

In the inverter device of the invention, when a power source connected to an electrolytic capacitor  5  is interrupted, a switching control circuit  6   a  controls switching elements of an inverter main circuit  7   a  to operate to supply a current to a load, thereby discharging charges of the electrolytic capacitor  5 , and an electrolytic capacitor electrostatic capacitance calculator  10   a  calculates the electrostatic capacitance of the electrolytic capacitor, on the basis of: a discharged charge amount which is obtained from an outflow current Ic from the electrolytic capacitor  5 , and a discharge time; and a discharge voltage ΔV which is a voltage drop from beginning of discharging of the electrolytic capacitor  5.

TECHNICAL FIELD

The present invention relates to an inverter device in which the life ofan electrolytic capacitor used in the inverter device can be determined.

BACKGROUND ART

As well known, an electrolytic capacitor is used in a main circuitsmoothing section of a voltage source inverter device. Such anelectrolytic capacitor has a limited life. Usually, when the designedtime of an inverter device has elapsed, it is therefore deemed that thelife of an electrolytic capacitor has expired, and the electrolyticcapacitor is replaced with a new one. However, the life of anelectrolytic capacitor is largely varied depending on the environment inwhich an inverter device is used. Therefore, there are a case where,although an electrolytic capacitor has not yet deteriorated, thecapacitor is replaced with a new one after the designed constant timehas elapsed, and an opposite case where, although an electrolyticcapacitor has deteriorated, the life of the capacitor is not deemed toexpire and hence the capacitor is not replaced because the designedconstant time has not yet elapsed.

An inverter device in which the life of an electrolytic capacitor usedin the inverter device can be determined is disclosed in Patentliterature 1 (JP-A-11-98854). The inverter device which is disclosed inPatent literature 1, and in which an electrolytic capacitor is used in amain circuit smoothing section comprises electrolytic capacitor lifedetermining means having: a discharge resistor which is connected inparallel to the electrolytic capacitor; a voltage detecting sectionwhich monitors the voltage between the terminals of the electrolyticcapacitor; and a judging section which, when a voltage supply to theelectrolytic capacitor is interrupted, measures the discharge timeconstant t (t=C•R) that is determined by the discharge resistance R andthe electrostatic capacitance C of the electrolytic capacitor, andwhich, when an electrostatic capacitance calculated from the measuredvalue exceeds an allowable range of the electrostatic capacitance of theelectrolytic capacitor that is previously obtained as a reference,judges that the life of the electrolytic capacitor has expired.

A capacitor life diagnostic apparatus which is developed for the purposeof accurately diagnosing the life of a capacitor is disclosed in Patentliterature 2 (JP-A-11-231008). In Patent literature 2, at a timing afterat least one of application of a power source voltage and stop of theapplication, the voltage between terminals of a capacitor is sampled atplural points, a time constant T is obtained from the sampled voltages,the capacitance C0 of the capacitor is calculated from an expression ofT/R with using a known resistance such as a resistance R of a rushcurrent preventing resistor that is connected to the capacitor of aswitching power source apparatus, and, when it is judged that thecapacitance C0 of the capacitor is equal to or smaller than thetheoretical worst capacitance Cr=T/(R±ΔR)+ΔC in which a change ±ΔR ofthe resistance R of the resistor and a change +ΔC of the capacitance ofthe capacitor caused by a change in ambient temperature are considered,it is judged that capacitance reduction occurs in the capacitor.

A main circuit power source discharging method in which a chargingresidual voltage of a capacitor is discharged as a load loss bysupplying a current to an electric motor through a closed circuitincluding the motor-without using a dedicated discharging circuit havinga discharge resistor is disclosed in Patent literature 3(JP-A-11-89264). In Patent literature 3, when a DC input power source toa main circuit is interrupted, the control is transferred from a motordriving control to a discharge mode, and a current is supplied to themotor by forming a closed circuit including the motor, whereby thecharging voltage of the capacitor is consumed and discharged as a loadloss or a torque energy of driving the motor and the load.

In Patent literature 1 or Patent literature 2, since a resistor (thedischarge resistor in Patent literature 1, and the rush currentpreventing resistor in Patent literature 2) is used in the estimation ofthe life of an electrolytic capacitor, there is a problem in that, whenthe resistance is varied by a change of the temperature surrounding theresistor, the estimation of the life of an electrolytic capacitor basedon the measurement of the time constant hardly maintains the accuracy.

There are other problems as follows. When a resistor to be used fordischarging of an electrolytic capacitor is set to have a small heatcapacity, the resistance is increased, and hence the output current ofthe capacitor is reduced. Therefore, the measurement of the dischargingtime is prolonged. By contrast, when the resistance is reduced in orderto shorten the measurement time of the discharging time, a larger poweris consumed in the resistor, and hence a resistor of a larger heatcapacity must be disposed. Therefore, the inverter device is increasedin size.

The resistor to be used for discharging of an electrolytic capacitor isplaced adjacent to the electrolytic capacitor. Even when heat of aquantity within a usable range of the resistor is generated, therefore,the temperature of the electrolytic capacitor itself is raised by theheat, thereby producing a further problem in that the heat causes theelectrostatic capacitance of the electrolytic capacitor to be varied.

The invention has been conducted in order to solve these problems. It isan object of the invention to obtain an inverter device in which thelife of an electrolytic capacitor can be accurately determined, and thetiming of replacement of the electrolytic capacitor can be exactlydetermined.

DISCLOSURE OF THE INVENTION

The inverter device of the invention is an inverter device having: anelectrolytic capacitor serving as a DC power source; an inverter maincircuit which has switching elements, and which converts a DC voltage ofthe electrolytic capacitor to an AC voltage; a switching control circuitwhich outputs a control signal for ON/OFF-controlling the switchingelements of the inverter main circuit; and an electrolytic capacitorelectrostatic capacitance calculator which calculates an electrostaticcapacitance of the electrolytic capacitor, wherein, when a power sourceconnected to the electrolytic capacitor is interrupted, the switchingcontrol circuit controls the switching elements of the inverter maincircuit to operate to supply a current to a load, thereby dischargingcharges of the electrolytic capacitor, and the electrolytic capacitorelectrostatic capacitance calculator calculates the electrostaticcapacitance of the electrolytic capacitor, on the basis of: a dischargedcharge amount which is obtained from an outflow current from theelectrolytic capacitor, and a discharge time; and a discharge voltagewhich is a voltage drop from beginning of discharging of theelectrolytic capacitor. Therefore, the electrostatic capacitance of theelectrolytic capacitor can be grasped as an absolute value, and the lifeof the electrolytic capacitor can be correctly predicted.

In the inverter device of the invention, when a power source connectedto the electrolytic capacitor is interrupted, the switching controlcircuit controls the switching elements of the inverter main circuit tooperate to supply a current to a load, thereby discharging charges ofthe electrolytic capacitor, and the electrolytic capacitor electrostaticcapacitance calculator calculates the electrostatic capacitance of theelectrolytic capacitor, on the basis of: a discharged charge amountwhich is obtained from an inverter output current that is detected by acurrent detector disposed on a side of an output of the inverter maincircuit, and ON/OFF states of the switching elements of the invertermain circuit; and a discharge voltage which is a voltage drop frombeginning of discharging of the electrolytic capacitor.

Therefore, a current detector which is disposed for each phase of ageneral purpose inverter in order to protect elements or control a loadcan be used as it is, and life prediction of an electrolytic capacitorcan be economically realized.

In the inverter device of the invention, when a power source connectedto the electrolytic capacitor is interrupted, the switching controlcircuit outputs a control signal for controlling an upper switchingelement and a lower switching element of a specific one phase of theinverter main circuit to ON/OFF-operate, upper switching elements ofother phases to be always turned OFF, and lower switching elements ofthe other phases to be always turned ON, to supply a current to a load,thereby discharging charges of the electrolytic capacitor, and theelectrolytic capacitor electrostatic capacitance calculator calculatesthe electrostatic capacitance of the electrolytic capacitor, on thebasis of: a discharged charge amount which is obtained from a currentflowing through the specific one phase in which the upper switchingelement and the lower switching element are ON/OFF-operated, and an ONtime in ON/OFF operations; and a discharge voltage which is a voltagedrop from beginning of discharging of the electrolytic capacitor.Therefore, it is not required to calculate the discharge time, and thelife of the electrolytic capacitor can be correctly predicted.

In the inverter device of the invention, when a power source connectedto the electrolytic capacitor is interrupted, the switching controlcircuit outputs a control signal for controlling an upper switchingelement and a lower switching element of a specific one phase of theinverter main circuit to ON/OFF-operate, upper switching elements ofother phases to be always turned OFF, and lower switching elements ofthe other phases to be always turned ON, to supply a current to a load,thereby discharging charges of the electrolytic capacitor, and theelectrolytic capacitor electrostatic capacitance calculator calculatesthe electrostatic capacitance of the electrolytic capacitor, on thebasis of; a discharged charge amount which is obtained from an outflowcurrent from the electrolytic capacitor, and an ON time in ON/OFFoperations; and a discharge voltage which is a voltage drop frombeginning of discharging of the electrolytic capacitor.

Therefore, it is possible to use a non-insulated current detector whichis economical.

In the inverter device of the invention, a comparator, and a currentcontroller for preventing an overcurrent on an output side of thecomparator are disposed on an input side of the switching controlcircuit, the comparator comparing a current command value which is usedin production of the control signal for ON/OFF-controlling the switchingelements of the inverter main circuit, with a current which flows outfrom the electrolytic capacitor, and which is to be used in theelectrolytic capacitor electrostatic capacitance calculator, or acurrent corresponding to the current which flows out from theelectrolytic capacitor. Even when the inductance of a winding is changedduring a load operation, therefore, the device can be controlled so thatan overcurrent is not produced, and the electrostatic capacitance of theelectrolytic capacitor can be correctly calculated.

The inverter device of the invention comprises an abnormal signal outputcircuit which outputs an abnormal signal when the electrostaticcapacitance of the electrolytic capacitor calculated by the electrolyticcapacitor electrostatic capacitance calculator is lower than a firstelectrostatic capacitance allowable value that is previously set.

Therefore, the user can easily determine the timing of replacement ofthe electrolytic capacitor.

In the abnormal signal output circuit in the inverter device of theinvention, a second electrostatic capacitance allowable value which islarger than the first electrostatic capacitance allowable value can beset, and, when the electrostatic capacitance of the electrolyticcapacitor calculated by the electrolytic capacitor electrostaticcapacitance calculator is lower than the second electrostaticcapacitance allowable value, an advance notice signal is output.

Therefore, the user can judge that the timing of replacement of theelectrolytic capacitor draws near, and prepare for a work of replacementof the electrolytic capacitor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing the configuration of an inverter device ofEmbodiment 1 of the invention.

FIG. 2 is a view showing the configuration of an inverter device ofEmbodiment 2 of the invention.

FIG. 3 is a view showing the configuration of an inverter device ofEmbodiment 3 of the invention.

FIG. 4 is a view showing an equivalent circuit of an inverter maincircuit of the inverter device of Embodiment 3 of the invention.

FIG. 5 is a view showing relationships between an electrolytic capacitoroutflow current Ic and a U-phase current Iu in the inverter device ofEmbodiment 3 of the invention.

FIG. 6 is a view showing the configuration of an inverter device ofEmbodiment 4 of the invention.

FIG. 7 is a view showing an equivalent circuit of an inverter maincircuit of the inverter device of Embodiment 4 of the invention.

FIG. 8 is a view showing the configuration of a switching controlcircuit in an inverter device of Embodiment 5 of the invention.

FIG. 9 is a view showing the configuration of an inverter device ofEmbodiment 6 of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION Embodiment 1

The configuration and process of an inverter device of Embodiment 1 ofthe invention will be described with reference to FIG. 1. Referring toFIG. 1, the inverter device 1 a is connected to a commercial powersource 2 through a breaker 3, a converter 4 converts the power-frequencyAC voltage of the commercial power source 2 to a DC voltage, and theconverted DC voltage is smoothed by an electrolytic capacitor 5.Switching elements constituting an inverter main circuit 7 a areON/OFF-controlled by a control signal from a switching control circuit 6a, whereby a DC power is converted to an AC power of predeterminedfrequency and voltage to drive an electric motor 8 serving as a load.

An electrolytic capacitor electrostatic capacitance calculator 10 acomprises a timer which measures a discharge time in discharging of theelectrolytic capacitor, and calculates the electrostatic capacitance ofthe electrolytic capacitor 5 on the basis of the voltage of theelectrolytic capacitor 5 detected by a voltage detector 11 a, an outflowcurrent from the electrolytic capacitor 5 detected by a current detector12 a, and the discharge time.

A method of predicting the life of the electrolytic capacitor 5 in theinverter device 1 a of Embodiment 1 will be described.

When the inverter device 1 a normally operates, the commercial powersource is supplied to the converter 4 through the breaker 3, and an ACcurrent which is controlled so as to have an arbitrary frequency by theinverter main circuit 7 a is output to the motor 8. When the commercialpower source 2 is interrupted by the breaker 3, charges remain to beaccumulated in the electrolytic capacitor 5.

When switching elements of the inverter main circuit 7 a are thenoperated to supply the charges accumulated in the electrolytic capacitor5 to the motor 8, the charges are rapidly discharged in the motor 8.Since the motor 8 has a capacity of several kW or several tens of kW,heat generation occurring in the rapid discharging exerts no influenceon the motor body. The motor 8 is not adjacent to the electrolyticcapacitor 5. Even when the temperature of the body of the motor 8 islargely changed, therefore, no influence is exerted on the electrolyticcapacitor 5.

During discharging, the electrolytic capacitor electrostatic capacitancecalculator 10 a obtains a discharge voltage ΔV which is a voltage dropfrom beginning of discharging, from the voltage of the electrolyticcapacitor 5 detected by the voltage detector 11 a. An electrolyticcapacitor outflow current Ic which is a current flowing out from theelectrolytic capacitor 5 detected by the current detector 12 a isintegrated over the discharge time t to obtain a discharged chargeamount. Then, the electrostatic capacitance C of the electrolyticcapacitor 5 is calculated from Expression (1) on the basis of thedischarge voltage ΔV and the discharged charge amount. $\begin{matrix}\begin{matrix}{{{Electrostatic}\quad{capacitance}\quad C} = {{discharged}\quad{charge}\quad{{amount}/}}} \\{{discharge}\quad{voltage}} \\{= {\int{{\left( {{Ic} \times t} \right)/\Delta}\quad V}}}\end{matrix} & (1)\end{matrix}$

As described above, the voltage and the current of the electrolyticcapacitor are detected, and the electrostatic capacitance of theelectrolytic capacitor 5 is then obtained. Therefore, the electrostaticcapacitance of the electrolytic capacitor 5 can be grasped as anabsolute value, and prediction of the life of the electrolytic capacitorcan be performed more correctly.

In the above, the example in which the timer which measures thedischarge time in discharging of the electrolytic capacitor is disposedin the electrolytic capacitor electrostatic capacitance calculator 10 ahas been described. Alternatively, the discharge time may be measured bya microcomputer or the like (not shown) which is incorporated into theinverter.

Embodiment 2

The configuration and process of an inverter device of Embodiment 2 ofthe invention will be described with reference to FIG. 2. In FIG. 2, 2to 5, 6 a, 7 a, and 8 denote the same components as those of FIG. 1, andtheir description is omitted. A voltage detector 11 b is configured soas to detect the voltage of the electrolytic capacitor 5, and obtain thedischarge voltage ΔV which is a voltage drop from beginning ofdischarging. A current detector 12 b is disposed for each phase of theoutput of the inverter main circuit 7.

An electrolytic capacitor electrostatic capacitance calculator 10 bobtains a discharged charge amount of the electrolytic capacitor 5, frominverter output currents Iu, Iv, Iw of the phases detected by thecurrent detector 12 b, and a control signal which is output from theswitching control circuit 6, and which is used for ON/OFF-controllingthe switching elements constituting the inverter main circuit 7, andcalculates the electrostatic capacitance of the electrolytic capacitor 5with using the discharged charge amount and the discharge voltage ΔV ofthe electrolytic capacitor 5 obtained by the voltage detector 11 b.

A method of calculating the charge amount and the electrostaticcapacitance of the electrolytic capacitor 5 will be described withrespect to (a) the case where they are obtained by an analog circuit,and (b) the case where they are obtained by using a microcomputer or thelike mounted in the inverter device.

(a) The case where the charge amount and the electrostatic capacitanceare obtained by an analog circuit.

When the inverter output currents of the phases are indicated by Iu, Iv,Iw, and a variable sign representing the switching status of each phaseis set so that, when an upper switching element is ON, sign=1, and, whena lower switching element is ON, sign=−1, the discharged charge amountof the electrolytic capacitor can be calculated from Expression (2). Inthis case, it is assumed that Iu, Iv, Iw are polarized and the directionfrom the inverter device to the load is set as positive. Dischargedcharge amount of electrolytic capacitor∫=(½)×{Iu×sign(u)+Iv×sign(v)+Iw×sign(w)} dt  (2)

The electrostatic capacitance C of the electrolytic capacitor 5 isobtained from Expression (3) with using the discharged charge amount ofthe electrolytic capacitor calculated by Expression (2), and thedischarge voltage ΔV which is a voltage drop from beginning ofdischarging. $\begin{matrix}\begin{matrix}{{{Electrostatic}\quad{capacitance}\quad C} = {\int{\left( {1/2} \right) \times \left\{ {{{Iu} \times {{sign}(u)}} +} \right.}}} \\{\left. {{{Iv} \times {{sign}(v)}} + {{Iw} \times {{sign}(w)}}} \right\}{{\mathbb{d}t}/\Delta}\quad V}\end{matrix} & (3)\end{matrix}$(b) The case where the charge amount and the electrostatic capacitanceare obtained by using a microcomputer or the like mounted in theinverter device.

When the inverter output currents of the phases are indicated by Iu, Iv,Iw, the ON times of upper switching elements of the phases are indicatedby UPon, VPon, WPon, and the switching period is T, the dischargedcharge amount of the electrolytic capacitor 5 can be calculated fromExpression (4).

Discharged charge amount of electrolytic capacitor=Σ(Iu×UPon/T+Iv×VPon/T+Iw×WPon/T)  (4)The electrostatic capacitance C of the electrolytic capacitor 5 isobtained from Expression (5).Electrostatic capacitance C=Σ(Iu×UPon/T+Iv×VPon/T+Iw×WPon/T)/ΔV  (5)

As described above, in the inverter device 1 b of Embodiment 2, theelectrostatic capacitance of the electrolytic capacitor 5 is calculatedwith using the ON/OFF states of the switching elements constituting theinverter main circuit 7 a.

The inverter device 1 a of Embodiment 1 is the example in which theoutflow current from the electrolytic capacitor 5 is detected by thededicated current detector 12 a. By contrast, the inverter device 1 b ofEmbodiment 2 uses the current detector 12 b which is attached for eachphase of the output of the inverter main circuit 7 in a general purposeinverter in order to protect elements or control a motor. Therefore, thedetection can be performed at low cost.

Embodiment 3

The configuration and process of an inverter device of Embodiment 3 ofthe invention will be described with reference to FIG. 3. In FIG. 3, 5,8, and 11 a denote the same components as those of FIG. 1, and theirdescription is omitted. When the commercial power source is interrupted,a switching control circuit 6 c outputs a control signal forON/OFF-controlling only switching elements of the U phase, and for, withrespect to the V and W phases, always turning OFF upper switchingelements, and always turning ON lower switching elements. A currentdetector 12 c is disposed on the output side of the U phase (thespecific one phase which is to be ON/OFF-operated when the commercialpower source is interrupted).

In an inverter main circuit 7 c, diode elements 13 are connected inparallel to the lower switching elements of the two phases which arealways turned ON, so that a circuit configuration is formed whichensures a current path in the state where, in the specific phase whichis to be ON/OFF-operated, the upper switching element is, turned OFF andthe lower switching element is turned ON. In the case where the upperswitching element of the U phase is turned OFF and the lower switchingelement is turned ON, the electrolytic capacitor outflow current Icwhich is a current flowing out from the electrolytic capacitor 5 flowsthrough the path of the lower switching element of the U phase → thelower diode elements of the V and W phases → the motor 8 → the lowerswitching element of the U phase. When the current flowing through the Uphase is indicated by Iu, the current flowing through the V phase by IV,and the current flowing through the W phase by Iw, the electrolyticcapacitor outflow current Ic is Ic=Iu IV+Iw.

An electrolytic capacitor electrostatic capacitance calculator 10 ccalculates the electrostatic capacitance C of the electrolytic capacitor5 with using the ON time Ton of the upper switching element of thespecific one phase which is to be ON/OFF-operated, a voltage drop ΔVbetween the terminals of the electrolytic capacitor 5, and the currentIu flowing through the U phase (=the electrolytic capacitor outflowcurrent Ic).

A process of calculating the electrostatic capacitance of theelectrolytic capacitor in the inverter device of Embodiment 3 of theinvention will be described with reference to FIGS. 3, 4, and 5.

When the commercial power source is interrupted, the switching elementsof the inverter main circuit 7 c are caused to perform theabove-mentioned ON/OFF operations, and discharging of the electrolyticcapacitor 5 is started.

FIG. 4 shows the path of the electrolytic capacitor outflow current Icwhen, in the case where the upper switching elements of the V and Wphases are always turned OFF, the lower switching elements are alwaysturned ON, and only the switching elements of the U phase areON/OFF-operated, the upper switching element of the U phase is turned ONand the lower switching element is turned OFF. The current flows throughthe path of the electrolytic capacitor 5 → the upper switching elementof the U phase → the motor 8 → the lower switching elements of the V andW phases (the current flows equally through the V and W phases)→ theelectrolytic capacitor 5.

With respect to the U-phase current Iu detected by the current detector12 c, as shown in FIG. 5 the electrolytic capacitor outflow current Icflows during the ON time Ton when the upper switching element of the Uphase is turned ON, and hence charges of the electrolytic capacitor 5are discharged so that the U-phase current Iu is increased. By contrast,during the time (Ts−Ton) which is equal to the carrier period Ts otherthan the ON time Ton, and in which the upper switching element of the Uphase is turned OFF and the lower switching element of the U phase isturned ON, charges of the electrolytic capacitor 5 are not discharged,and the U-phase current IU is reduced. Only during the ON time Tonobtained from an ON command which is output from the switching controlcircuit 6 c to the upper switching element of the U phase, therefore,the charges accumulated in the electrolytic capacitor 5 are discharged,so that the discharge time=the ON time Ton.

The electrolytic capacitor electrostatic capacitance calculator 10 cthen multiplies the U-phase current Iu detected by the current detector12 c with the discharge time (=the ON time Ton obtained from the ONcommand which is output from the switching control circuit 6 c to theupper switching element of the U phase), and integrates the product.From the voltage Vdc of the electrolytic capacitor 5 detected by thevoltage detector 11 a, the difference between the voltage of theelectrolytic capacitor at the beginning of discharging and that of theelectrolytic capacitor during discharging is obtained, and the dischargevoltage ΔV which is a voltage drop is obtained.

The electrostatic capacitance C of the electrolytic capacitor 5 isobtained by Expression (6) from: the discharged charge amount of theelectrolytic capacitor 5 which is obtained by integrating the U-phasecurrent Iu over the ON time Ton obtained from the ON command output tothe upper switching element of the U phase; and the discharge voltageΔV.Electrostatic capacitance C=∫(Iu×Ton)/ΔV  (6)

In the above, the example in which only the U phase is ON/OFF operated,and, with respect to the V and W phases, the upper switching elementsare always turned OFF and the lower switching elements are always turnedON has been described. The same effects can be attained also when thespecific one phase which is to be ON/OFF-operated is the V phase or theW phase. In this case, the current detector 12 c is disposed for the Vphase or the W phase.

Embodiment 4

The configuration and process of an inverter device of Embodiment 4 ofthe invention will be described with reference to FIG. 6. In FIG. 6, 5,6 c, 8, 11 a, and 13 denote the same components as those of FIG. 3, andtheir description is omitted. A current detector 12 d is disposedbetween a lower switching element of an inverter main circuit 7 d andthe electrolytic capacitor 5.

An electrolytic capacitor electrostatic capacitance calculator 10 dcalculates the electrostatic capacitance C of the electrolytic capacitor5 with using the ON time Ton of the upper switching element of thespecific one phase which is to be ON/OFF-operated, a voltage drop ΔVbetween the terminals of the electrolytic capacitor 5, and theelectrolytic capacitor outflow current Ic.

A process of calculating the electrostatic capacitance of theelectrolytic capacitor in the inverter device of Embodiment 4 of theinvention will be described with reference to FIGS. 6 and 7.

When the commercial power source is interrupted, the switching elementsof the inverter main circuit 7 d are caused to perform theabove-mentioned ON/OFF operations, and discharging of the electrolyticcapacitor 5 is started.

FIG. 7 shows the path of the electrolytic capacitor outflow current Icwhen, in the case where the upper switching elements of the V and Wphases are always turned OFF, the lower switching elements are alwaysturned ON, and only the switching elements of the U phase areON/OFF-operated, the upper switching element of the U phase is turned ONand the lower switching element is turned OFF. The current flows throughthe path of the electrolytic capacitor 5 → the upper switching elementof the U phase the motor 8 → the lower switching elements of the V and Wphases (the current flows equally through the V and W phases) → theelectrolytic capacitor 5. When the current flowing through the U phaseis indicated by Iu, the current flowing through the V phase by Iv, andthe current flowing through the W phase by Iu, the electrolyticcapacitor outflow current Ic is Ic=Iu=Iv+Iw.

The electrolytic capacitor electrostatic capacitance calculator 10 dmultiplies the electrolytic capacitor outflow current Ic detected by thecurrent detector 12 d with the discharge time (=the ON time Ton obtainedfrom the ON command which is output from the switching control circuit 6c to the upper switching element of the U phase), and integrates theproduct. The electrolytic capacitor electrostatic capacitance calculator10 d calculates the electrostatic capacitance C of the electrolyticcapacitor 5 by Expression (7) from: the ON time Ton of the upperswitching element of the specific one phase which is to beON/OFF-operated; and the discharge voltage ΔV between the terminals ofthe electrolytic capacitor 5; and the electrolytic capacitor outflowcurrent Ic.Electrostatic capacitance C=∫(Ic×Ton)/ΔV  (7)

The above Expression (7) for calculating the electrostatic capacitance Cof the electrolytic capacitor 5 is obtained by replacing the U-phasecurrent Iu of Expression (6) for calculating the electrostaticcapacitance C of the electrolytic capacitor 5 in Embodiment 3, with theelectrolytic capacitor outflow current Ic.

In Embodiment 3, the current detector 12 c is disposed in the output ofthe specific one phase (the U phase in FIG. 3) which is to beON/OFF-operated when the commercial power source is interrupted, and, inthe case where the switching elements are controlled so that only theswitching elements of the U phase are ON/OFF-operated and, with respectto the V and W phases, the upper switching elements are always turnedOFF and the lower switching elements are always turned ON, theelectrolytic capacitor outflow current Ic=the current Iu flowing throughthe U phase. Therefore, the current Iu flowing through the U phase isused in place of the electrolytic capacitor outflow current Ic.

In the method in which the current to be used for calculating theelectrostatic capacitance of the electrolytic capacitor is detected bythe current detector 12 c disposed in the output of the inverter asshown in FIG. 3, when a power source (not shown) for the controlcircuits is set to be equal to the potential of the point A, insulationis required, and hence an insulated current detector must be used.

In Embodiment 4, the current detector 12 d is disposed between theelectrolytic capacitor 5 and the lower switching elements of theinverter main circuit 7 d. Even when the power source for the controlcircuits is set to be equal to the potential of the point A, therefore,a non-insulated current detector can be used as the current detector.For example, an economical shunt resistor may be used.

In the above, the example in which the current detector 12 d fordetecting the electrolytic capacitor outflow current IC is disposedbetween the electrolytic capacitor 5 and the lower switching elements ofthe inverter main circuit 7 d has been described. The same effects canbe attained also when a current detector is disposed between the lowerswitching element of the U phase and that of the V phase.

Embodiment 5

A switching control circuit in an inverter device of Embodiment 5 of theinvention will be described with reference to FIG. 8.

In the switching control circuit 14 in the inverter device of Embodiment5, a comparator 15 which compares a current command value i* with acurrent detection value i, and a current controller 16 for preventing anovercurrent are disposed on the input side of a switching controlcircuit 6 (6 a, 6 c) which outputs a control signal forON/OFF-controlling the switching elements of an inverter main circuit(not shown), thereby forming a current loop. Even when the inductance ofa winding is changed during a load operation, therefore, the device canbe controlled so that an overcurrent is not produced, and theelectrostatic capacitance of the electrolytic capacitor can be correctlycalculated.

Embodiment 6

The configuration and process of an inverter device of Embodiment 6 ofthe invention will be described with reference to FIG. 9. In the figure,2 to 5, 6 a, 7 a, 8, 10 a, 11 a, and 12 a denote the same components asthose of FIG. 1, and their description is omitted.

In the inverter device le of Embodiment 6, an abnormal signal outputcircuit 16 outputs an abnormal signal when the electrostatic capacitanceof the electrolytic capacitor 5 calculated by the electrolytic capacitorelectrostatic capacitance calculator 10 a is lower than a firstelectrostatic capacitance allowable value. When the electrostaticcapacitance of the electrolytic capacitor 5 is lower than a secondelectrostatic capacitance allowable value which is larger than the firstelectrostatic capacitance allowable value, the abnormal signal outputcircuit 17 outputs an advance notice signal.

In the inverter device le of Embodiment 6, when the electrostaticcapacitance of the electrolytic capacitor 5 is lower than the firstelectrostatic capacitance allowable value, the abnormal signal isoutput. Therefore, the user can easily determine the timing ofreplacement of the electrolytic capacitor. The second electrostaticcapacitance allowable value which is larger than the first electrostaticcapacitance allowable value for determining the timing of replacement ofthe electrolytic capacitor is previously set, and, when theelectrostatic capacitance of the electrolytic capacitor 5 is lower thanthe second electrostatic capacitance allowable value, the advance noticesignal is output. Therefore, the user can judge that the timing ofreplacement of the electrolytic capacitor draws near, and prepare for awork of replacement of the electrolytic capacitor. Consequently, thestop time of the inverter device during a work of replacing theelectrolytic capacitor can be minimized.

In Embodiments 1 to 6, the examples in which the electric motor 8 isused as the load have been described. Even when the load is an inductiveload such as an inductive heating apparatus or an ozone generator, theelectrostatic capacitance of the electrolytic capacitor 5 can besimilarly obtained, and correct life prediction can be performed.

Industrial Applicability

As described above, in the inverter device of the invention, the life ofan electrolytic capacitor can be accurately determined, and the timingof replacement of the electrolytic capacitor can be exactly determined.Therefore, the inverter device is suitable as an inverter device whichis to be installed in a place where it is difficult to perform aninspection for measuring the life of an electrolytic capacitor from theoutside.

1. An inverter device comprising: an electrolytic capacitor serving as aDC power source; an inverter main circuit which has switching elements,and which converts a DC voltage of said electrolytic capacitor to an ACvoltage; a switching control circuit which outputs a control signal forON/OFF-controlling said switching elements of said inverter maincircuit; and an electrolytic capacitor electrostatic capacitancecalculator which calculates an electrostatic capacitance of saidelectrolytic capacitor, wherein when a power source connected to saidelectrolytic capacitor is interrupted, said switching control circuitcontrols said switching elements of said inverter main circuit tooperate to supply a current to a load, thereby discharging charges ofsaid electrolytic capacitor, and said electrolytic capacitorelectrostatic capacitance calculator calculates the electrostaticcapacitance of said electrolytic capacitor, on the basis of a dischargedcharge amount which is obtained from an outflow current from saidelectrolytic capacitor, and a discharge time, and a discharge voltagewhich is a voltage drop from beginning of discharging of saidelectrolytic capacitor.
 2. An inverter device comprising: anelectrolytic capacitor serving as a DC power source; an inverter maincircuit which has switching elements, and which converts a DC voltage ofsaid electrolytic capacitor to an AC voltage; a switching controlcircuit which outputs a control signal for ON/OFF-controlling saidswitching elements of said inverter main circuit; and an electrolyticcapacitor electrostatic capacitance calculator which calculates anelectrostatic capacitance of said electrolytic capacitor, wherein when apower source connected to said electrolytic capacitor is interrupted,said switching control circuit controls said switching elements of saidinverter main circuit to operate to supply a current to a load, therebydischarging charges of said electrolytic capacitor, and saidelectrolytic capacitor electrostatic capacitance calculator calculatesthe electrostatic capacitance of said electrolytic capacitor, on thebasis of a discharged charge amount which is obtained from an inverteroutput current that is detected by a current detector disposed on a sideof an output of said inverter main circuit, and ON/OFF states of saidswitching elements of said inverter main circuit, and a dischargevoltage which is a voltage drop from beginning of discharging of saidelectrolytic capacitor.
 3. An inverter device comprising; anelectrolytic capacitor serving as a DC power source; an inverter maincircuit which has switching elements, and which converts a DC voltage ofsaid electrolytic capacitor to an AC voltage; a switching controlcircuit which outputs a control signal for ON/OFF-controlling saidswitching elements of said inverter main circuit; and an electrolyticcapacitor electrostatic capacitance calculator which calculates anelectrostatic capacitance of said electrolytic capacitor, wherein when apower source connected to said electrolytic capacitor is interrupted,said switching control circuit outputs a control signal for controllingan upper switching element and a lower switching element of a specificone phase of said inverter main circuit to ON/OFF-operate, upperswitching elements of other phases to be always turned OFF, and lowerswitching elements of said other phases to be always turned ON, tosupply a current to a load, thereby discharging charges of saidelectrolytic capacitor, and said electrolytic capacitor electrostaticcapacitance calculator calculates the electrostatic capacitance of saidelectrolytic capacitor, on the basis of a discharged charge amount whichis obtained from a current flowing through said specific one phase inwhich said upper switching element and said lower switching element areON/OFF-operated, and an ON time in ON/OFF operations, and a dischargevoltage which is a voltage drop from beginning of discharging of saidelectrolytic capacitor.
 4. An inverter device comprising: anelectrolytic capacitor serving as a DC power source; an inverter maincircuit which has switching elements, and which converts a DC voltage ofsaid electrolytic capacitor to an AC voltage; a switching controlcircuit which outputs a control signal for ON/OFF-controlling saidswitching elements of said inverter main circuit; and an electrolyticcapacitor electrostatic capacitance calculator which calculates anelectrostatic capacitance of said electrolytic capacitor, wherein when apower source connected to said electrolytic capacitor is interrupted,said switching control circuit outputs a control signal for controllingan upper switching element and a lower switching element of a specificone phase of said inverter main circuit to ON/OFF-operate, upperswitching elements of other phases to be always turned OFF, and lowerswitching elements of said other phases to be always turned ON, tosupply a current to a load, thereby discharging charges of saidelectrolytic capacitor, and said electrolytic capacitor electrostaticcapacitance calculator calculates the electrostatic capacitance of saidelectrolytic capacitor, on the basis of a discharged charge amount whichis obtained from an outflow current from said electrolytic capacitor,and an ON time in ON/OFF operations, and a discharge voltage which is avoltage drop from beginning of discharging of said electrolyticcapacitor.
 5. An inverter device according to any one of claims 1 to 4,wherein a comparator, and a current controller for preventing anovercurrent on an output side of said comparator are disposed on aninput side of said switching control circuit, said comparator comparinga current command value which is used in production of the controlsignal for ON/OFF-controlling said switching elements of said invertermain circuit, with a current which flows out from said electrolyticcapacitor, and which is to be used in said electrolytic capacitorelectrostatic capacitance calculator, or a current corresponding to thecurrent which flows out from said electrolytic capacitor.
 6. An inverterdevice according to any one of claims 1 to 5, wherein said devicecomprises an abnormal signal output circuit which outputs an abnormalsignal when the electrostatic capacitance of said electrolytic capacitorcalculated by said electrolytic capacitor electrostatic capacitancecalculator is lower than a first electrostatic capacitance allowablevalue that is previously set.
 7. An inverter device according to claim6, wherein, in said abnormal signal output circuit, a secondelectrostatic capacitance allowable value which is larger than the firstelectrostatic capacitance allowable value can be set, and, when theelectrostatic capacitance of said electrolytic capacitor calculated bysaid electrolytic capacitor electrostatic capacitance calculator islower than the second electrostatic capacitance allowable value, anadvance notice signal is output.